Communication systems are designed to transfer information between two or more devices over a transmission medium in the presence of interference or transmission influences. Intersymbol interference (ISI) is one well-known information-compromising influence in which transmitted symbols become elongated and interfere with adjacently transmitted symbols. This spreading or “smearing” of symbols is generally caused by bandwidth limitation and interference associated with all communication components and mediums and/or multipath propagation within those mediums. Because ISI has the same effect as noise, signal integrity may be compromised or made less reliable.
Furthermore in an optical transmission system, transmission components such as direct modulated lasers (DML) have different rise and fall times. A DML performs nonlinear conversion of input current into output optical intensity. In a multilevel phase amplitude modulation (e.g., PAM4) application, this nonlinearity of DML can cause the three PAM4 inner eyes, as plotted in an “eye diagram,” to misalign. Fiber transmission can further make the PAM4 signal distorted.
An adaptive filtering process, referred to as equalization, is often used to flatten the frequency response of a communication components and medium and mitigate the effects of ISI. One specific filtering architecture used to flatten the frequency response is a Decision Feedback Equalizer (DFE) which may contain both a forward filter and a feedback filter. The performance of the DFE is generally limited by non-idealities such as frequency response, noise, nonlinearity, and mismatch associated with its data path and the data it operates on.
A receiver with a single sampling phase and a linear equalizer cannot compensate for such nonlinearity. As a result, the link performance exhibits a high bit-error rate. What is needed is a method and system to compensate for such nonlinearity of a DML and other components to improve the receiver sensitivity as well as an error floor of a system such as a PAM4 system.